Optical tomographic imaging apparatus

ABSTRACT

An optical tomographic imaging apparatus includes a first lens, an optical path branching unit, a second lens and a scanning unit disposed on an optical path of measurement light with which an object to be examined is irradiated. The second lens and the scanning unit are disposed in such a manner that an angle at which the measurement light scanned by the scanning unit is incident on the optical path branching unit satisfies a wavelength separation characteristic of 90% or higher at a wavelength of observation light of the object to be examined in a wavelength band of the measurement light.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical tomographic imagingapparatus for use in an ophthalmologic medical care and the like.

2. Description of the Related Art

Currently, various ophthalmologic apparatuses using optical apparatusesare known. For example, various apparatuses such as an anterior eyeportion imaging apparatus, a fundus camera, and a confocal scanninglaser ophthalmoscope (SLO) are used as optical apparatuses for observinga subject's eye. Among them, an optical tomographic imaging apparatusbased on optical coherence tomography (OCT) utilizing multi-wavelengthlightwave interference is an apparatus that can acquire a tomographicimage of a sample at a high resolution, and is becoming an apparatusessential for clinics specialized in retinas as an ophthalmologicapparatus. Hereinafter, this apparatus will be referred to as an OCTapparatus.

The OCT apparatus emits measurement light, which is low-coherent light,to the sample, and can measure backscattering light from this sample ata high sensitivity by using an interference system or an interferenceoptical system. The low-coherent light is characterized in that atomographic image can be acquired at a high resolution by increasing awavelength width thereof. Further, the OCT apparatus can acquire atomographic image at a high resolution by scanning the measurement lighton the sample. Therefore, the OCT apparatus can acquire a tomographicimage of a retina on a fundus of a subject's eye, and is widely used inan ophthalmologic diagnosis and the like of a retina.

On the other hand, generally, the OCT apparatus as an ophthalmologicapparatus is provided with a fundus observation optical system, ananterior eye observation optical system, and the like for an alignmentadjustment between the apparatus and the subject's eye. The OCTapparatus is constructed by using light beams having differentwavelengths in the respective optical systems and separating thewavelengths with use of a wavelength separation unit such as a dichroicmirror, to allow the OCT apparatus to be used together with theseoptical systems.

Now, suppose that a light source for OCT emits light having a centralwavelength of 855 nm, and a wavelength band from approximately 805 nm toapproximately 905 nm with a wavelength bandwidth of approximately 100nm. On the other hand, a light source for OCT discussed in JapanesePatent Application Laid-Open No. 2011-11052 emits light having a centralwavelength of 840 nm, and a wavelength band from approximately 815 nm toapproximately 865 nm with a wavelength bandwidth of approximately 50 nm.Further, suppose that a light source configured to produce light havinga wavelength of 780 nm is used as a light source of an SLO. In thiscase, an interval between the wavelength of the light source of the SLOand an end of the wavelength band of the light source for OCT isapproximately 35 nm (815 nm-780 nm) in the technique discussed inJapanese Patent Application Laid-Open No. 2011-11052. On the other hand,in the case where the wavelength bandwidth is approximately 100 nm, thisinterval is approximately 25 nm (805 nm-780 nm). In this manner, in thecase of the wavelength bandwidth of approximately 100 nm, the wavelengthbandwidth of the light source for OCT is wider than that of thetechnique discussed in Japanese Patent Application Laid-Open No.2011-11052, thereby leading to a reduction in the interval between thewavelength of the light source of the SLO and the end of the wavelengthband of the light source for OCT.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, an optical tomographicimaging apparatus, which is configured to acquire a tomographic image ofan object to be examined based on light generated by combining returnlight from the object to be examined irradiated by measurement light viaa first lens and reference light corresponding to the measurement light.The optical tomographic imaging apparatus includes a scanning unitdisposed on an optical path of the measurement light and configured toscan the measurement light on the object to be examined, a second lensdisposed on the optical path of the measurement light between thescanning unit and the first lens, and an optical path branching unitdisposed between the first lens and the second lens and configured tobranch the optical path of the measurement light to form an observationoptical path for observing the object to be examined therefrom. Thesecond lens and the scanning unit are disposed in such a manner that anangle at which the measurement light scanned by the scanning unit isincident on the optical path branching unit satisfies a wavelengthseparation characteristic of 90% or higher at a wavelength ofobservation light of the object to be examined in a wavelength band ofthe measurement light.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an outline of a configuration of an opticaltomographic imaging apparatus according to an exemplary embodiment ofthe present invention.

FIG. 2 illustrates exemplary optical paths of a light flux of lightincident on a pupil of a subject's eye, in an optical tomographicimaging apparatus according to the exemplary embodiment of the presentinvention.

FIG. 3 illustrates an example of how measurement light is scanned on asubject's eye by an X scanner in an X direction in the opticaltomographic imaging apparatus.

FIG. 4 illustrates exemplary representations of an image of an anterioreye, a two-dimensional image of a fundus, and a B-scan image displayedon a monitor in the optical tomographic imaging apparatus.

FIGS. 5A, 5B, and 5C illustrate what kind of characteristic a wavelengthtransmission characteristic of a dichroic mirror is in the opticaltomographic imaging apparatus according to the exemplary embodiment ofthe present invention.

FIG. 6 illustrates an angle range Δθ at which the measurement light isincident on the dichroic mirror when a scanning unit is located offsetfrom a focal position of a lens by a distance ΔX in the opticaltomographic imaging apparatus.

DESCRIPTION OF THE EMBODIMENTS

A change in an incident angle of measurement light to a dichroic mirrortypically leads to a change in a wavelength separation characteristic (achange in a wavelength band of light transmittable through the dichroicmirror). Therefore, if a low-coherence light source configured to emitlight having a wider wavelength bandwidth than that of a conventionaltechnique is used as a light source for OCT, the accuracy for wavelengthseparation should be further improved compared to the conventionaltechnique because of a shorter interval between a wavelength of a lightsource of an SLO and an end of the wavelength band of the light sourcefor OCT.

According to an exemplary embodiment of the present invention, anoptical tomographic imaging apparatus includes a scanning unit disposedon an optical path of measurement light with which an object to beexamined is irradiated via a first lens, and a second lens disposedbetween the scanning unit and the first lens. Then, the second lens andthe scanning unit are disposed in such a manner that an angle at whichthe measurement light scanned by the scanning unit is incident on anoptical path branching unit is maintained substantially unchanged duringscanning. To that end, the scanning unit is disposed at a focal positionof the second lens. As a result, even when the measurement light isscanned by the scanning unit, it is possible to reduce a change in awavelength separation characteristic of the optical path branching unit.Therefore, even if a low-coherent light source configured to emit lighthaving a wider wavelength bandwidth than that of the conventionaltechnique is used as the light source for OCT, it is possible to improvethe accuracy of wavelength separation for separating the wavelength ofthe light source for OCT and the wavelength of the light source of theSLO. For example, it is possible to reduce a variation in atransmittance (or a reflectance) of a predetermined wavelength to beseparated by the dichroic mirror, which occurs due to a difference inthe angle at which the measurement light is incident on the dichroicmirror. Wavelength separation characteristic refers to, for example, aratio of wavelength transmission to wavelength reflection achieved bythe dichroic mirror.

Now, it is diagnostically desirable that an OCT tomographic image of afundus of a subject's eye allows well observation of a thinner membranethan a layer such as a junction between photoreceptor inner and outersegments (IS/OS), and an external limiting membrane (ELM). For realizingthis, an axial resolution of 3 μm should be achieved in the OCTtomographic image. Then, the axial resolution depends on the wavelengthbandwidth of the OCT light source, and the wavelength bandwidth of theOCT light source should be approximately 100 nm to achieve the axialresolution of 3 μm.

As illustrated in FIG. 5A, the second lens and the scanning unit shouldbe disposed in such a manner that the angle at which the measurementlight scanned by the scanning unit is incident on the optical pathbranching unit can satisfy a wavelength separation characteristic of 90%or higher at a wavelength in the wavelength band of the measurementlight that is closest to a wavelength of observation light of the objectto be examined, to maintain approximately 100 nm as the wavelengthbandwidth of the OCT light source. FIGS. 5A, 5B, and 5C illustratewavelength transmission characteristics of the dichroic mirror.

For example, suppose that, as illustrate in FIG. 5C, the second lens andthe scanning unit are disposed in such a manner that the angle at whichthe measurement light is incident on the dichroic mirror is largelydeviated from an angle of 45 degrees, when the wavelength band of theOCT light source as an example of the light source of the measurementlight is from approximately 805 nm to approximately 905 nm. In thiscase, suppose that this angle is large to a degree at which thewavelength transmission characteristic (a transmittance) of the dichroicmirror falls below 90% at approximately 805 nm, which is close to awavelength 780 nm of the SLO light source as an example of thewavelength of the observation light of the object to be examined. Inthis case, the wavelength band of the OCT light source is narrowed to,for example, from approximately 810 nm to approximately 905 nm, wherebythe wavelength bandwidth thereof falls below approximately 100 nm. As aresult, the axial resolution also falls below 3 μm (the value thereofincreases), whereby the image quality of the OCT tomographic image isalso deteriorated. Therefore, the second lens and the scanning unitshould be disposed in such a manner that the wavelength transmissioncharacteristic of 90% or higher can be satisfied at approximately 805nm.

Further, as illustrated in FIG. 5B, it is desirable that the second lensand the scanning unit are disposed in such a manner that a wavelengthseparation characteristic of 8% or lower is satisfied at the wavelengthof the observation light of the object to be examined. For example,suppose that the wavelength transmission characteristic (thetransmittance) of the dichroic mirror exceeds 8% at the wavelength 780nm of the SLO light source, which is an example of the light source ofthe observation light of the object to be examined. In this case, returnlight for the SLO is introduced into an OCT optical system by a lightamount large to a degree at which this light is observed in an OCTtomographic image as a noise. Therefore, it is desirable that the secondlens and the scanning unit are disposed in such a manner that thewavelength transmission characteristic of 8% or lower is satisfied atapproximately 780 nm.

To meet the above-described conditions, as illustrated in FIG. 6, it isdesirable that the second lens and the scanning unit are disposed insuch a manner that the angle at which the measurement light is incidenton the optical path branching unit can become equal to or larger than 44degrees and smaller than 46 degrees (Δθ=45 degrees±1 degrees), althoughthis also depends on features of the optical path branching unit. FIG. 6illustrates the angle Δθ with which the measurement light is incident onthe dichroic mirror when the scanning unit is located offset from afocal position of the lens by a distance ΔX. Further, examples of thefeatures of the optical path branching unit include a material of a filmof the dichroic mirror, the number of films of the dichroic mirror, anda thickness of the film of the dichroic mirror. In this case, it isdesirable that the second lens 101-3 and a central position 127 of thescanning unit are disposed in such a manner that the position of thescanning unit is located within a range of ±f/10 mm (±ΔX) from the focalposition of the second lens in an optical axis direction, where frepresents a focal length of the second lens.

Next, an optical path desirable for an anterior eye observation opticalsystem, and an optical path desirable for a fundus observation opticalsystem will be described.

<Optical Path Desirable for Anterior Eye Observation Optical System>

First, an optical path desirable for the anterior eye observationoptical system will be described.

As described above, the fundus observation optical system and the OCToptical system are configured in such a manner that return lightgenerated by a return of the measurement light scanned by the scanningunit from a subject's eye is incident on the second lens at an angle ofapproximately 45 degrees via the dichroic mirror. On the other hand, inthe anterior eye observation optical system, light from an anterior eyeportion of the subject's eye enters a fourth lens (for example, a lens141 illustrated in FIG. 1), which is a lens of the anterior eye portionoptical system, while being scattered via the dichroic mirror.Therefore, in the anterior eye portion optical system, as the distancefrom the first lens (for example, a lens 101-1 illustrated in FIG. 1) tothe fourth lens increases, the light is scattered more widely, therebyleading to an increase in size of the fourth lens. Constructing a relaylens to reduce the scattering results in an increase in the number ofoptical members, thereby leading to an increase in the size of theanterior eye observation optical system.

For this reason, it is desirable that the anterior eye observationoptical system is disposed on an optical path close to the first lens.For example, as illustrated in FIG. 1, it is desirable that the anterioreye observation optical system is disposed on an optical path L3, whichis a transmission optical path of a first dichroic mirror 102.

<Optical Path Desirable for Fundus Observation Optical System>

Next, an optical path desirable for the fundus observation opticalsystem will be described.

First, it is desirable that the measurement light for the SLO in thefundus observation optical system has a larger beam diameter when themeasurement light is incident on the anterior eye portion, than themeasurement light for OCT in the OCT optical system. The reason thereofis as follows. In the SLO optical system, the return light from thesubject's eye is detected by a single detector 116 after being formedinto a ring shape by a prism 118 such as a holed mirror. In the SLOoptical system, a light amount reduces according to the formation of thereturn light into a ring shape, whereby it is desirable to increase thebeam diameter as much as possible. On the other hand, the OCT opticalsystem acquires a tomographic image with use of interference light at aposition close to a coherence gate. Therefore, when a tomographic imageof a fundus is captured, the tomographic image does not contain an imageof normal reflection on the anterior eye portion away from a range wherethe tomographic image is acquired. Therefore, the return light from thesubject's eye does not have to be formed into a ring shape in the OCToptical system, unlike the SLO optical system, whereby the measurementlight for OCT can have a smaller beam diameter than the measurementlight for the SLO. The SLO optical system may be configured to form themeasurement light before application to the subject's eye into a ringshape.

In this case, as the beam diameter increases, a range where the lens isirradiated by the beam increases in a radial direction of the lens,whereby the beam is scattered more widely. Therefore, the measurementlight for the SLO is more largely affected by an astigmatism on atransmission optical path of the dichroic mirror than the measurementlight for OCT. For this reason, it is desirable that the fundusobservation optical system is disposed on a reflection optical path ofthe dichroic mirror. For example, as illustrated in FIG. 1, it isdesirable that the fundus observation optical system is disposed on anoptical path L2, which is a reflection optical path of the firstdichroic mirror 102 and a reflection optical path of a second dichroicmirror 103.

In this manner, as illustrated in FIG. 1, it is desirable that theanterior eye observation optical system, the fundus observation opticalsystem, and the OCT optical system are disposed on the optical path L3,the optical path L2, and an optical path L1, respectively.

In the following description, the present exemplary embodiment will bedescribed with reference to the accompanying drawings. The same elementsare identified with the same reference numerals throughout the presentdisclosure.

<Configuration of Apparatus>

A configuration of the optical tomographic imaging apparatus (OCTapparatus) according to the present exemplary embodiment will bedescribed with reference to FIG. 1. The optical tomographic imagingapparatus includes an optical head 900 and a spectrometer 180. Theoptical tomographic imaging apparatus acquires a tomographic image ofthe object to be examined based on light generated by combining thereturn light from the object to be examined irradiated by themeasurement light via the scanning unit, and reference lightcorresponding to this measurement light.

First, an internal configuration of the optical head 900 will bedescribed. The optical head 900 includes measurement optical systems forcapturing an image of an anterior eye of a subject's eye 100, atwo-dimensional image of a fundus of the subject's eye 100, and atomographic image of the subject's eye 100. The lens 101-1, which is anobjective lens and an example of the first lens, is disposed so as toface the subject's eye 100. Further, an optical path is branched by thefirst dichroic mirror 102 and the second dichroic mirror 103, which arethe optical path branching unit. Herein, the first dichroic mirror 102and the second dichroic mirror 103 are non-limiting examples of anoptical path branching unit. Other examples of the optical pathbranching unit may include a prism, a half-silvered mirror, a half-waveplate, or the like. Regardless of how the optical path branching unit isimplemented, the optical path is branched into the measurement opticalpath L1 of the OCT optical system, the fundus observation optical pathand fixation lamp optical path L2, and the anterior eye portionobservation optical path L3. These optical paths L1, L2 and L3 arebranched according to each wavelength band.

<Optical Path L1: Measurement Optical Path of OCT Optical System>

The optical path L1 is the measurement optical path of the OCT opticalsystem, which is separated according to the wavelength from the light ofthe anterior eye observation optical system on the optical path L3 bythe first dichroic mirror 102, and is further separated according to thewavelength from the measurement light of the fundus observation opticalsystem on the optical path L2 by the second dichroic mirror 103. The OCToptical system is used to capture a tomographic image of the fundus ofthe subject's eye 100. More specifically, the optical path L1 is used toacquire an interference signal for forming a tomographic image. A lens101-3, which is an example of the second lens, a mirror 121, and thescanning unit are disposed on the optical path L1. The scanning unitincludes an X scanner 122-1, which is an example of a first scanningunit, and a Y scanner 122-2, which is an example of a second scanningunit. A non-limiting example of the X scanner 122-1 and Y scanner 122-2includes a scanning galvanometer. One-dimensional (1D) ortwo-dimensional (2D) galvanometer optical scanners may be used. Othernon-limiting examples may include 1D or 2D MEMS (micro-electromechanicalmirrors) scanning mirrors. Regardless of the implementation, the Xscanner 122-1 and the Y scanner 122-2 scan the light on the fundus ofthe subject's eye 100 in an X direction (a main scanning direction alsoreferred to as a first direction), and a Y direction (a sub scanningdirection also referred to as a second direction intersecting with thefirst direction). In FIG. 1, an optical path between the X scanner 122-1and the Y scanner 122-2 is formed in a direction in parallel with theplane of the sheet of FIG. 1, but actually, this optical path is formedin a direction perpendicularly to the plane of the sheet of FIG. 1.

<Optical Path L2: Optical Path of Fundus Observation Optical System>

The optical path L2 is the optical path of the fundus observationoptical system, which is separated according to the wavelength from themeasurement light of the OCT optical system on the optical path L1 bythe second dichroic mirror 103. Among a lens 101-2, a focusing lens 111,and a lens 112, the focusing lens 111 is driven by a motor(not-illustrated) for a focusing adjustment for a fixation lamp andfundus observation (not-illustrated).

First, on the optical path L2, a light source 115 (the light source ofthe SLO) for fundus observation generates light having a wavelength of780 nm. Further, an X scanner 117-1, which is an example of a firstobservation scanning unit, and a Y scanner 117-2, which is an example ofa second observation scanning unit, are disposed on the optical path L2to scan the light emitted from the light source 115 for fundusobservation on the fundus of the subject's eye 100. The lens 101-2,which is an example of a third lens, is disposed so as to have a focalposition around a central position between the X scanner 117-1 and the Yscanner 117-2. The X scanner 117-1 includes a polygon mirror to scan thelight in the X direction at a high speed. Further, the X scanner 117-1may include a resonant mirror. Further, the single detector 116 includesan avalanche photodiode (APD), and detects the light scattered by andreflected from the fundus. The prism 118 is a prism to which a holedmirror or a hollow mirror is evaporated, and separates the illuminationlight from the light source 115 for fundus observation and the returnlight from the fundus.

Further, a dichroic mirror (not-illustrated) may be further provided,and a light-emitting diode (LED) or the like may be further provided asa light source of the fixation lamp (not-illustrated). In this case, thelight source of the fixation lamp is disposed on the SLO light sourceside relative to the scanning unit for observation. Due to thisarrangement, the scanning unit for observation is also used as ascanning unit for visual fixation, by which a scanning fixation lamp canbe formed. In this case, this scanning fixation lamp can work well byusing a control unit (not-illustrated) that performs control in such amanner that the light source of the fixation lamp is turned on whenlight from the light source of the fixation lamp is scanned at aposition desired by an examiner. Turning on and turning off the lightsource of the fixation lamp may be replaced with opening and closing ashutter disposed on this optical path.

The optical path L2 may be a line scanning SLO (a line SLO) that scans aline beam in a single direction by using a cylindrical lens or the like,instead of the above-described point scanning SLO that two-dimensionallyscans a spot to acquire a two-dimensional image of the fundus. Further,the optical path L2 may be configured to perform infrared observation byusing a two-dimensional charge coupled device (CCD) sensor, instead ofusing the scanning unit. More specifically, the optical path L2 may beconfigured to include a CCD sensor for fundus observation, instead ofthe X scanner 117-1 and the Y scanner 117-2, to acquire atwo-dimensional image of the fundus of the subject's eye 100. In thiscase, the two-dimensional CCD sensor is configured to detect awavelength of the illumination light (not-illustrated) for fundusobservation, in particular, around 780 nm.

Further, the fixation lamp on the optical path L2 may be configured insuch a manner that the examiner prompts visual fixation of an examineeto a desired position by generating visual light by a display for visualfixation such as a liquid-crystal display, and changing a lightingposition on the display for visual fixation. In this case, the displayfor visual fixation is disposed closer to a third dichroic mirror 104relative to the scanning unit for observation.

<Optical Path L3: Optical Path of Anterior Eye Observation OpticalSystem>

The optical path L3 is the optical path of the anterior eye observationoptical system where the lens 141, and an infrared CCD sensor 142 foranterior eye observation are disposed. The infrared CCD sensor 142 has asensitivity to a wavelength of the illumination light (not-illustrated)for anterior eye observation, in particular, around 970 nm.

<Position Optically Conjugate with Anterior Eye Portion: SubstantiallyCentral Position Between X and Y Scanners Coincides with Focal Positionof Lens>

Now, conjugate relationships between the eye position and the opticalpath L1 and the optical path L2, and a light flux of the eye will bedescribed with reference to FIG. 2. The optical tomographic imagingapparatus is configured in such a manner that a position conjugate witha predetermined portion such as the anterior eye portion of thesubject's eye 100 is located between the first scanning unit and thesecond scanning unit. The present exemplary embodiment can be realizedas long as at least one of the optical path L1 and the optical path L2is configured in this manner.

First, referring back to FIG. 2, on the optical path L1, a scannercentral position 127 between the X scanner 122-1 and the Y scanner122-2, and a pupil position 128 (the anterior eye portion) of thesubject's eye 100 are in an optically conjugate relationship. Morespecifically, the optical system of the optical head 900 is designed insuch a manner that the X and Y scanners 122-1 and 122-2 configured toscan the measurement light for OCT in the X and Y directions and theanterior eye portion of the subject's eye are set in an opticallyconjugate relationship, when the optical head 900 and the subject's eye100 are aligned with each other. As a result, it is possible to reducevignetting of the measurement light on the anterior eye portion of thesubject's eye 100.

Further, the lens 101-1, the lens 101-3, and the X scanner 122-1 and theY scanner 122-2 (or the scanner central position 127) are disposed insuch a manner that a light flux of the measurement light scanned by thescanning unit is substantially collimated between the lens 101-1 and thelens 101-3. According to this configuration, an optical path for which ameasurement light deflection unit is set as an object point issubstantially collimated between the lens 101-1 and the lens 101-3.Then, the scanner central position 127 coincides with a focal positionof the lens 101-3. Due to this configuration, it is possible tosubstantially maintain angles at which the measurement light is incidenton the first dichroic mirror 102 and the second dichroic mirror 103,even when the X scanner 122-1 and the Y scanner 122-2 scan themeasurement light. As a result, even when the measurement light for OCTis scanned by the X and Y scanners 122-1 and 122-2, it is possible toreduce changes in the wavelength separation characteristics of thedichroic mirrors 102 and 103, whereby it is possible to improve theaccuracy of wavelength separation by the dichroic mirrors 102 and 103.

Further, on the optical path L2, a scanner central position 119 betweenthe X scanner 117-1 and the Y scanner 117-2, and the pupil position 128of the subject's eye 100 are also in a conjugate relationship. Further,the lens 101-2 and the scanner central position 119 (the X scanner 117-1and the Y scanner 117-1) are disposed in such a manner that a light fluxis substantially collimated between the lens 101-1 and the lens 101-2.According to this configuration, an optical path for which a measurementlight deflection unit is set as an object point is substantiallycollimated between the lens 101-1 and the lens 101-2. Then, the scannercentral position 119 coincides with a focal position of the lens 101-2.Due to this configuration, it is possible to substantially maintainangles with which the measurement light is incident on the firstdichroic mirror 102 and the second dichroic mirror 103, even when the Xscanner 117-1 and the Y scanner 117-2 scan the measurement light. As aresult, even when the measurement light for the SLO is scanned by the Xand Y scanners 117-1 and 117-2, it is possible to reduce changes in thewavelength separation characteristics of the dichroic mirrors 102 and103, whereby it is possible to improve the accuracy of wavelengthseparation by the dichroic mirrors 102 and 103.

Further, the optical path L1 and the optical path L2 are configured toshare the lens 101-1, and it is desirable that the lens 101-2 and thelens 101-3 are configured by lenses having similar shapes and made ofsimilar materials. As a result, it is possible to establish matchingoptical systems from the subject's eye 100 to the respective X and Yscanners 122-1, 122-2, 117-1, and 117-2 on the optical path L1 and theoptical path L2, whereby it is possible to uniform opticalcharacteristics on the optical paths L1 and L2. Therefore, it becomespossible to reduce an error in a measurement.

Now, as illustrated in FIG. 2, assume that 0 represents an angle formedby the light flux of the pupil to the pupil of the subject's eye 100, θ1represents an angle formed by the light flux of the pupil to the scannercentral position 127, and θ2 represents an angle formed by the lightflux of the pupil to the scanner central position 119. In other words,the optical tomographic imaging apparatus is configured to provide theangles θ1 and θ2 to the light beams with use of the scannersrespectively to acquire the angle θ formed by the light flux of thepupil on both the optical path L1 and the optical path 12.

Further, as one of the optical characteristics, an optical magnificationof the scanner central position 119 to the pupil position 128 and anoptical magnification of the scanner central position 127 to the pupilposition 128 can be also uniformed on the optical path L1 and theoptical path L2. As a result, relationships between scan angles of the Xand Y scanners 122-1, 122-2, 117-1, and 117-2 on the respective opticalpaths L1 and L2, and illumination positions on the fundus of thesubject's eye 100 can be uniformed on the optical paths L1 and L2,whereby the angles θ1 and θ2 can become equal to each other. Due to thisarrangement, it becomes possible to reduce an error between therespective scanning positions.

<Position Optically Conjugate with Fundus: Focusing Adjustment>

Further, the optical system of the optical head 900 is designed in sucha manner that a fiber end 126 for introducing the measurement light tothe measurement optical path and the fundus of the subject's eye 100 areset into an optically conjugate relationship by performing a focusingadjustment, when the X and Y scanners 122-1 and 122-2 and the anterioreye portion are in an optically conjugate relationship. The focusinglens 123 and a lens 124 are provided adjacent to the fiber end 126, andthe focusing lens 123, which is one of them, is driven in directionsindicated by a double-headed arrow by a motor (not-illustrated) toperform a focusing adjustment. The focusing adjustment is performed bymaking an adjustment in such a manner that light emitted from themeasurement light source 126, which is the fiber end, is imaged on thefundus. The focusing lens 123, which is an example of a focusing unit,is disposed between the measurement light source 126, and the X scanner122-1 and the Y scanner 122-2, which are the measurement lightdeflection unit. This configuration eliminates the necessity of movingthe larger lens 101-3 and a fiber 125-2 connected to the measurementlight source 126.

Now, for example, U.S. Pat. No. 5,537,162 discusses a configuration thatmaintains a constant angle as an incident angle at which a beam isincident on a dichroic mirror even when the beam is scanned by placing abeam scanner on a back focal plane of a lens (lens corresponding to thelens 101-3 in the present exemplary embodiment). Further, U.S. Pat. No.5,537,162 discusses that the beam scanner and the lens are integrallydriven during execution of a focusing adjustment for a fundus of asubject's eye. In this case, the lens (lens corresponding to the lens101-3 in the present exemplary embodiment) with the beam scanner placedon the back focal plane thereof tends to have a large size to introducescanning light of the beam scanner. Therefore, a driving mechanismtherefor is complicated, because the beam scanner and the large-sizedlens should be integrally moved. Further, since they are integrallymoved, a measurement light source in an optically conjugate relationshipwith a fundus position should be moved at the same time. If thismeasurement light source is an optical fiber end, an optical fibershould be moved, whereby a change may occur in a polarized state.Therefore, according to the present exemplary embodiment, as describedabove, the focusing lens 123 is disposed between the X and Y scanners122-1 and 122-2 that scan the measurement light for OCT in the X and Ydirections, and the fiber end 126 that emits the measurement light forOCT (or an optical coupler 125 that branches light into the measurementlight and the reference light). If the focusing position is changed bymoving the lens 101-1 in the optical axis direction, this also causes achange in the optically conjugate relationship between the X and Yscanners 122-1 and 122-2 and the anterior eye portion, wherebyvignetting of the measurement light may occur on an iris of the anterioreye portion and the like.

With this focusing adjustment, an image of the measurement light source126 can be formed on the fundus of the subject's eye 100, and the returnlight from the fundus of the subject's eye 100 can be efficientlyreturned to the fiber 125-2 via the measurement light source 126.Further, a focusing adjustment can be performed with use of a focusinglens 111 on the optical path L2 in a similar manner.

<Configuration of OCT Optical System>

Next, configurations of an optical path of light emitted from a lightsource 130 illustrated in FIG. 1, a reference optical system, and thespectrometer 180 will be described. A Michelson interference system isformed by the light source 130, a mirror 153, a dispersion compensationglass 152, the optical coupler 125, optical fibers 125-1 to 125-4, alens 151, and the spectrometer 180. The optical fibers 125-1 to 125-4form a single-mode optical fiber by being connected to the opticalcoupler 125 to be integrated all together.

The light emitted from the light source 130 is transmitted to theoptical coupler 125 via the optical fiber 125-1, and is divided into themeasurement light emitted to the optical fiber 125-2 and the referencelight emitted to the optical fiber 125-3 via the optical coupler 125.The fundus of the subject's eye 100 is irradiated with the measurementlight, which is an observation target, via the above-described opticalpath of the OCT optical system, and reaches the optical coupler 125 viathe same optical path by being reflected or scattered by a retina.

On the other hand, the reference light reaches the mirror 153 and isreflected thereby after being transmitted via the optical fiber 125-3,the lens 151, and the dispersion compensation glass 152 inserted tomatch dispersion of the measurement light and dispersion of thereference light. Then, the reference light reaches the optical coupler125 by returning through the same optical path.

The measurement light and the reference light are combined by theoptical coupler 125, thereby producing interference light. Interferenceoccurs when an optical path length of the measurement light and anoptical path length of the reference light become substantially equal.The mirror 153 is held in such a manner that its position can beadjusted in the optical axis direction by a motor and driving mechanism(not-illustrated), and can match the optical path length of thereference light to the optical path length of the measurement light,which varies depending on the subject's eye 100. The interference lightis guided to the spectrometer 180 via the optical fiber 125-4.

The spectrometer 180 includes a lens 181, a diffraction grating 182, alens 183, and a line sensor 184. The interference light emitted from theoptical fiber 125-4 is dispersed by the diffraction grating 182 afterbeing substantially collimated via the lens 181, and is imaged on theline sensor 184 by the lens 183.

Next, the light source 130 will be described. The light source 130 is asuper luminescent diode (SLD), which is a representative low-coherentlight source. The central wavelength is 855 nm, and the wavelengthbandwidth is approximately 100 nm. The wavelength bandwidth is animportant parameter, because it affects a resolution of an acquiredtomographic image in the optical axis direction. Further, the SLD isselected in the present example as the type of the light source, but thelight source 130 may be any light source that can emit low-coherentlight and can be also realized by amplified spontaneous emission (ASE)and the like. A suitable central wavelength is near infrared light inconsideration of the fact that the optical tomographic imaging apparatusis used to measure a subject's eye. Further, it is desirable that thecentral wavelength is as a small wavelength as possible, because itaffects a lateral resolution of an acquired tomographic image. For bothreasons, 855 nm is selected as the central wavelength.

The present exemplary embodiment uses a Michelson interferometer as theinterferometer, but may use a Mach-Zehnder interferometer. It isdesirable to, according to a difference in light amount between themeasurement light and the reference light, use a Mach-Zehnderinterferometer if their light amounts are largely different and to use aMichelson interferometer if their light amounts are relatively slightlydifferent.

<Method for Capturing Tomographic Image>

The optical tomographic imaging apparatus can capture a tomographicimage of a desired portion on the fundus of the subject's eye 100 bycontrolling the X scanner 122-1 and the Y scanner 122-2.

FIG. 3 illustrates how the subject's eye 100 is irradiated withmeasurement light 201, and the measurement light 201 is scanned on afundus 202 in the X direction. Information corresponding to apredetermined number of times of imaging is captured from an imagingrange on the fundus 202 in the X direction by the line sensor 184.Luminance distribution on the line sensor 184 that is acquired at acertain position in the X direction is transformed by fast Fouriertransformation (FFT). Linear luminance distribution acquired by FFT isconverted into density or color information to be displayed on amonitor, and this converted image is referred to as an A-scan image.Further, a two-dimensional image formed by arranging a plurality ofA-scan images is referred to as a B-scan image. A plurality of B-scanimages can be acquired by capturing a plurality of A-scan images toconstruct a single B-scan image, and then moving a scanning position inthe Y direction and performing scanning in the X direction again. Theplurality of B-scan images or a three-dimensional tomographic imageconstructed from the plurality of B-scan images is displayed on themonitor, whereby the examiner can use the image for a diagnosis of thesubject's eye 100.

FIG. 4 illustrates examples of an anterior eye image 210, a fundustwo-dimensional image 211, and a B-scan image 212, which is atomographic image, displayed on a monitor 200. The anterior eye image210 is an image processed and displayed from an output of the infraredCCD sensor 142. The fundus two-dimensional image 211 is an imageprocessed and displayed from an output of a CCD 114. Then, the B-scanimage 212 is an image constructed by performing the above-describedprocessing from an output of the line sensor 184.

As described above, according to the present exemplary embodiment, inthe optical tomographic imaging apparatus, the focusing unit (thefocusing lens 123 and the driving mechanism (not-illustrated))configured to perform a focusing adjustment that targets the subject'seye 100 is disposed between the measurement light deflection unit (the Xand Y scanners 122-1 and 122-2) configured to deflect the measurementlight, and the measurement light source 126. Further, the first lens(the lens 101-1) and the second lens (the lens 101-3) are disposed onthe measurement optical path between the measurement light deflectionunit (the X and Y scanners 122-1 and 122-2) and the subject's eye 100,and the optical path branching unit (the first dichroic mirror 102 andthe second dichroic mirror 103) is disposed between the first lens andthe second lens.

In other words, the focusing lens 123 is disposed between the fiber end126 as the measurement light source and the X and Y scanners 122-1 and122-2 as the measurement light deflection unit, which eliminates thenecessity of moving the large lens 101-3, the fiber 125-2 connected tothe measurement light source 126, and the like, leading tosimplification of the driving mechanism. Further, because the fiber end126 does not have to be moved, it is possible to provide an opticaltomographic imaging apparatus capable of maintaining a polarized state.Further, according to the present exemplary embodiment, in the opticaltomographic imaging apparatus, the first lens (the lens 101-1) and thesecond lens (the lens 101-3), and the measurement light deflection unit(the X and Y scanners 122-1 and 122-2) are positionally adjusted andarranged in such a manner that the light is collimated on themeasurement optical path between the first lens (the lens 101-1) and thesecond lens (the lens 101-3). As a result, it is possible to maintainconstant angles as the incident angles with which the beam is incidenton the first and second dichroic mirrors 102 and 103, thereby improvingthe accuracy of wavelength separation.

The present exemplary embodiment has been described, targeting asubject's eye. However, the present invention may scan light on not onlya subject's eye but also another object to be examined, like a humanbody such as skin and an internal organ, and can be employed for notonly an ophthalmologic apparatus but also an imaging apparatus such asan endoscope.

OTHER EMBODIMENTS

Embodiments of the present invention can also be realized by a computerof a system or apparatus that reads out and executes computer executableinstructions recorded on a storage medium (e.g., non-transitorycomputer-readable storage medium) to perform the functions of one ormore of the above-described embodiment(s) of the present invention, andby a method performed by the computer of the system or apparatus by, forexample, reading out and executing the computer executable instructionsfrom the storage medium to perform the functions of one or more of theabove-described embodiment(s). The computer may comprise one or more ofa central processing unit (CPU), micro processing unit (MPU), or othercircuitry, and may include a network of separate computers or separatecomputer processors. The computer executable instructions may beprovided to the computer, for example, from a network or the storagemedium. The storage medium may include, for example, one or more of ahard disk, a random-access memory (RAM), a read only memory (ROM), astorage of distributed computing systems, an optical disk (such as acompact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™),a flash memory device, a memory card, and the like.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2013-095622 filed Apr. 30, 2013, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An optical tomographic imaging apparatusconfigured to acquire a tomographic image of an object to be examinedbased on light generated by combining return light from the object to beexamined irradiated by measurement light via a first lens, and referencelight corresponding to the measurement light, the optical tomographicimaging apparatus comprising: a scanning unit disposed on an opticalpath of the measurement light, and configured to scan the measurementlight on the object to be examined; a second lens disposed on theoptical path of the measurement light between the scanning unit and thefirst lens; and an optical path branching unit disposed between thefirst lens and the second lens, and configured to branch from theoptical path of the measurement light to an observation optical path forobserving the object to be examined, wherein the second lens and thescanning unit are disposed in such a manner that an angle at which themeasurement light scanned by the scanning unit is incident on theoptical path branching unit satisfies a wavelength separationcharacteristic of 90% or higher at a wavelength of observation light ofthe object to be examined in a wavelength band of the measurement light.2. The optical tomographic imaging apparatus according to claim 1,wherein the second lens and the scanning unit are disposed in such amanner that the angle at which the measurement light scanned by thescanning unit is incident on the optical path branching unit satisfiesthe wavelength separation characteristic of 90% or higher at thewavelength of the observation light of the object to be examined in thewavelength band of the measurement light, and also satisfies awavelength separation characteristic of 8% or lower at the wavelength ofthe observation light of the object to be examined.
 3. The opticaltomographic imaging apparatus according to claim 2, wherein the secondlens and the scanning unit are disposed in such a manner that the angleat which the measurement light scanned by the scanning unit is incidenton the optical path branching unit is equal to or larger than 44degrees, and smaller than 46 degrees.
 4. The optical tomographic imagingapparatus according to claim 3, wherein the second lens and the scanningunit are disposed in such a manner that a position of the scanning unitis located within a range of ±f/10 mm from a focal position of thesecond lens in an optical axis direction, where f represents a focallength of the second lens.
 5. The optical tomographic imaging apparatusaccording to claim 1, wherein the wavelength band of the measurementlight is from approximately 805 nm to approximately 905 nm, wherein thewavelength of the observation light of the object to be examined isapproximately 780 nm, and wherein the second lens and the scanning unitare disposed in such a manner that the wavelength separationcharacteristic of 90% or higher is satisfied at the wavelength ofapproximately 805 nm.
 6. The optical tomographic imaging apparatusaccording to claim 1, wherein the optical path of the measurement lightis located on a transmission optical path of the optical path branchingunit, wherein the observation optical path is located on a reflectionoptical path of the optical path branching unit, and wherein thewavelength separation characteristic is a wavelength transmissioncharacteristic that allows transmission of the wavelength band of themeasurement light.
 7. The optical tomographic imaging apparatusaccording to claim 6, wherein a beam diameter of the measurement lightis smaller than a beam diameter of measurement light for observation onthe observation optical path.
 8. The optical tomographic imagingapparatus according to claim 1, wherein the object to be examined is asubject's eye, and wherein the scanning unit is disposed on a positionsubstantially conjugate with an anterior eye portion of the subject'seye.
 9. The optical tomographic imaging apparatus according to claim 1,wherein the object to be examined is a subject's eye, and wherein thescanning unit includes first and second scanning units configured toscan the measurement light on a fundus of the subject's eye indirections crossing each other; the optical tomographic imagingapparatus further comprising: third and fourth scanning units configuredto be disposed on the observation optical path and configured to scanlight for observation of the fundus on the fundus in directions crossingeach other; and a third lens configured to be disposed on theobservation optical path and configured to be disposed between saidthird and fourth scanning units and said optical path branching unit,wherein said second lens and said first and second scanning units aredisposed such that a position between said first and second scanningunits is approximately conjugate to an anterior ocular segment of theeye, and wherein said third lens and said third and fourth scanningunits are disposed such that a position between said third and fourthscanning units is approximately conjugate to the anterior ocularsegment.
 10. The optical tomographic imaging apparatus according toclaim 9, further comprising: a first light source configured to bedisposed on the observation optical path; and a second light sourceconfigured to be disposed on the first light source side relative to thethird and fourth scanning units, wherein the third and fourth scanningunits are used as scanning units for visual fixation.
 11. The opticaltomographic imaging apparatus according to claim 9, wherein said first,second and third lenses and said first, second, third and fourthscanning units are disposed such that an optical magnification at acentral position of said first and second scanning units conjugate tothe anterior ocular segment is equal to an optical magnification at acentral position of said third and fourth scanning units with respect tothe anterior ocular segment.
 12. The optical tomographic imagingapparatus according to claim 9, further comprising: a dividing unitconfigured to divide light generated from a light source into themeasurement light and the reference light, wherein the light sourcegenerates light in which the central wavelength is about 855 nm and thewavelength band width is about 100 nm, and wherein the light forobservation of the fundus is light having a wavelength of about 780 nm.13. The optical tomographic imaging apparatus according to claim 9,further comprising: a second optical path branching unit configured tobe disposed between the first lens and said second lens and configuredto make the optical path of the measurement light branch off to anoptical path of an anterior eye observation optical system forobservation of an anterior ocular segment of the eye, by separatinglight having a wavelength different from a wavelength of the measurementlight, wherein the optical path of the anterior eye observation opticalsystem is disposed on a transmission optical path of said second opticalpath branching unit, and wherein the optical path of the measurementlight is disposed on a reflection optical path of said second opticalpath branching unit.
 14. The optical tomographic imaging apparatusaccording to claim 13, wherein said second optical path branching unitis disposed at the first lens side with respect to said optical pathbranching unit, and wherein the observation optical path and the opticalpath of the measurement light are disposed on a reflection optical pathof said second optical path branching unit.